The present invention demonstrates an adjustable monolithic multi-wavelength laser diode array which are formed by a plurality of diode lasers on a substrate.
With the flourishing development of computer and the rapid growth of internet applications, people have higher demanding for data transmission and signal bandwidth. Since optical fiber has the advantages of wide bandwidth and low loss, it becomes the main medium of network transmission. Nowadays local area network has already been promoted from ethernet to high speed ethernet, and then to Gigabit ethernet. Televisions, networks, and cable televisions will associate together with the liberation of telecommunication. In order to provide these high speed and flexible services, optical networks using optical fiber as transmitting media becomes necessary. The fast increase in the demands for wide network bandwidth has stimulated the rapid growth of WDM fiber networks since WDM technology can greatly increase the bandwidth of the optical fiber networks. Multi-wavelength emitters are used in WDM networks to provide multi-wavelength optical signal. Usually they are formed of multi-wavelength laser array or combined from a plurality of tunable lasers. The key point of the wide-spreading usage of WDM lies in the performance and price of its components, including multi-wavelength light source, wavelength multiplexer/demultiplexer, optical filter, and multi-wavelength receivers, etc. The goal of the present invention is to combine the advantages of multi-wavelength laser array and tunable lasers to a simple method of fabricating high performance tunable laser arrays.
The required single frequency lasers (e.g. DFB laser) and tunable lasers (e.g. DBR laser)for high speed optical networks and long-distance communication have been mass produced. With the reduction of their cost, their applications are getting more widespread. Most of the multi-wavelength laser arrays and tunable lasers are made of DFB or DBR lasers currently. The fabrication procedures of the above laser arrays are usually very complicated and the wavelength spacing between lasers is difficult to control precisely, so they are not easy to commercialized. At present there are various technologies proposed for realizing tunable lasers for WDM systems. Among them, DBR, sampled grating DBR, and SSG etc. are more promising. Most of the embodiments of the present invention adopt the basic elements of the sampled grating DBR lasers of which the manufacturing technology is already very matured.
In response to the rapid growth of the volume of communication, WDM advances toward the direction of high density i.e., small channel spacing. To rule the devices and systems used in dense WDM networks, and to avoid the problem of wavelength mismatch between the light source and the interconnecting optical devices, ITU has established the standards of wavelength position and spacing in multi-wavelength (frequency) networks, therefore the wavelength preciseness of all optoelectronic devices used in dense WDM networks are getting more stringent. The multi-wavelength light sources in this type of networks must possess stable and accurate wavelengths. The wavelength spacing between channels in a laser array is difficult to control precisely and the yield is low with most of prior approaches for fabricating monolithic multi-wavelength laser arrays. If a plurality of independent tunable lasers are assembled to form a multi-wavelength light source, the cost is high and the size is large. The present invention is to resolve the above problems of multi-wavelength laser arrays for providing high performance and economic multi-wavelength emitters required in WDM network. Since the proposed tunable laser arrays have both of the advantages of multi-wavelength laser arrays and tunable lasers. The emitted wavelength of each laser can be adjusted and controlled by a simple method, therefore the wavelength and the wavelength spacing of a laser array can be set precisely.
The present invention combines both of the advantages of multi-wavelength laser arrays and tunable lasers to fabricate tunable multi-wavelength laser arrays and the detailed description of the present invention is as follows.
(1) prior technology: the prior technology related to the present invention can be classified into three categories
1. Multi-wavelength Laser Arrays
Current multi-wavelength laser arrays are mainly composed of a plurality of DFB lasers or DBR lasers formed on the same chip, every laser in the array must have a different wavelength, usually neighboring lasers have the same wavelength spacing. The output beam of DFB lasers and DBR lasers is of single wavelength and have high sidemode suppression ratio, hence they are suitable for the elements of the multi-wavelength laser arrays; one advantage of DBR lasers is that their wavelength can be tuned. In general, the wavelength of each laser is determined by the Bragg wavelength of gratings in a laser array formed of DFB lasers or DBR lasers. Express Bragg wavelength by xcexO, then xcexO can be written as: xcexO=2neffxcex9
where neff is the effective refractive index of the waveguide; xcex9 is the period of the grating. From the above equation, it can be seen that to vary the laser wavelength of the lasers in an array, the period of the grating or the effective refractive index of the waveguide has to be varied.
Hence there are several types of the approaches for fabricating multi-wavelength laser arrays in the prior technology as follows:
(1) Using multiple holographic exposure to vary the grating period of each laser, as shown by M. G. Young et al. in IEEE Photonics Technology Letters, Vol. 5, pp. 908-910 (1993).
(2) Making the angle between the waveguide of each laser and the grating different in order to vary the effective period of the grating, as shown by A. M. Sarangan et al. in IEEE Photonics Technology Letters, Vol. 8, pp. 1435-1437 (1996).
(3) Using e-beam lithography to directly write the grating of each laser so that they have different periods, as shown by T. P. Lee et al. in Journal of Lightwave Technology, Vol. 14, pp. 967-976 (1996).
(4) Using selective area growth technology to vary the effective index of every waveguide, as shown by T. Sasaki et al. in Journal of Crystal Growth, pp. 846-851 (1994).
(5) Using multiple etching technology to vary the thickness of every waveguide so that they have different effective index, as proposed by F. Delorme etal. In IEEE Photonics Technology Letters Vol. 8, pp. 867-869 (1996). p0 (6) Varying the width of every waveguide on the mask so that they have different effective index, as shown by G. P. Li, T. Makino et al. in IEEE Photonics Technology Letters Vol. 8, pp. 22-24 (1996).
The method (1) needs a special holographic exposure system, and it is very time-consuming. The performance of each laser by method (2) is not uniform, and the range of settable wavelength is limited. Fabricating the laser array by method (3) is very time consuming and not suitable for mass production. It is difficult to fabricate laser arrays which have uniform wavelength spacing and accurate wavelength by method (4), and the yield is also difficult to control. the laser array fabricated by (5) or (6) is prone to be affected by the process and it""s difficult to control the wavelength, and the range of settable wavelength is also limited.
2. Tunable Laser
The following are the most often used tunable semiconductor lasers:
(1) DBR-S. Wang, xe2x80x9cPrinciples of Distributed feedback and Distributed Bragg Lasersxe2x80x9d IEEE Journal of Quantum Electronics, QE-10, pp.413-30 427, (1974)
(2) DFB-H. Kogelnik, C. V. Shank, APPl. Phys. Lett. 18, pp. 152 (1971).
(3) SGDBR-L. A. coldren, xe2x80x9cMULTI-SECTION TUNABLE LASER WITH DIFFERING MULTI-ELEMENT MIRRORSxe2x80x9d U.S. Pat. No.: 4,896,325, Date of issue at Jan. 23 (1990).
(4) GACC-R. C. Alferness et al., xe2x80x9cBroadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57-nm tuning rangexe2x80x9d Appl. Phys. Lett., vol. 60, no. 26, 29, pp.3209-3211 (1992).
(5) SSG-DBR-Y Tohmori et al., xe2x80x9cUltrawide wavelength tuning with single longitudinal mode by super structure grating (SSG) DBR lasers, xe2x80x9d 13th IEEE laser conf. ""92, O-6 (1992).
(6) Y junction-S. Wang et al., IEEE Journal of Quantum Electronics, QE-17, pp.453 (1981).
The former two methods have smaller wavelength setting range ( less than 15 nm), and the wavelength range of lasers fabricated by the other methods can be as long as several 10""s nm or even more than 100 nm.
3. Semiconductor Laser Mirrors with a Plurality of Reflectivity Peaks
(1) sampled grating: as proposed by V. Jayaraman et al. in Proceedings of the sixth International Conference on Indium Phosphide and Related Materials (IPRM ""94), 1994.
(2) super structure grating: as proposed by Y. Tohmori et al. in Electronics Letters, Vol. 29, pp. 352-354 (1993).
(3) binary superposition grating: as proposed by I. A. Avrutsky et al. in IEEE Journal of Quantum Electronics, Vol. 34, pp. 729-741 (1998).